Mechanistic link between lidocaine block and inactivation probed by outer pore mutations in the rat μ1 skeletal muscle sodium channel

1. Mutations that disrupt Na+ channel fast inactivation attenuate lidocaine (lignocaine)-induced use dependence; however, the pharmacological role of slower inactivation processes remains unclear. In Xenopus oocytes, tryptophan substitution in the outer pore of the rat skeletal muscle channel mu 1-W402) alters partitioning among fast- and slow-inactivated states. We therefore examined the effects of W402 mutations on lidocaine block. 2. Recovery from inactivation exhibited three kinetic components (I-F, fast; I-M, intermediate; I-S, slow). The effects of W402A and W402S on I-F and I-S differed, but both mutants (with or without beta(1) subunit coexpression) decreased the amplitude of I-M. In wild-type channels, lidocaine imposed a delayed recovery component with intermediate kinetics, and use-dependent block was attenuated in both W402A and W402S. 3. To examine the pharmacological role of I-S relative to I-M, drug-exposed beta(1)-coexpressed channels were subjected to 2 min depolarizations. Lidocaine had no effect on sodium current (I-Na) after a 1s hyperpolarization interval that allowed recovery from I-M but not I-S, suggesting that lidocaine affinity for I-S is low. 4. Both W402 mutations reduced occupancy of I-M in drug-free conditions, and also induced resistance to use-dependent block. We propose that lidocaine-induced use dependence may involve an allosteric conformational change in the outer pore.